Seismic surveys image or map the subsurface of the earth by imparting acoustic energy into the ground and recording the reflected energy or “echoes” that return from the rock layers below. Each time the energy source is activated it generates a seismic signal that travels into the earth, is partially reflected, and, upon its return, may be recorded at many locations on the surface as a function of travel time.
A land seismic survey typically uses one of two energy sources to generate the down going seismic signal: either an explosive source or a vibrational source. Of particular interest for purposes of the instant disclosure is the use of seismic vibrator. A seismic vibrator generally takes the form of a truck or other vehicle that has a base plate that can be brought into contact with the earth. A reaction mass in association with a baseplate is driven by a hydraulic system to produce vibratory motion that travels downward into the earth via the base plate. This truck-mounted vibrator sends a seismic sweep into the earth and then the collected data is correlated and stored. This method is also known by the name “vibroseis” or the vibroseis method.
The receivers that are used to detect the returning seismic energy for the land survey usually take the form of sensors like geophones or accelerometers. The returning seismic energy is acquired from a continuous signal representing displacement, velocity or acceleration that may be represented as an amplitude variation as a function of time.
A seismic survey may be designed that uses multiple vibrators, each being activated simultaneously so that the receivers and recording instruments capture a composite signal with contributions from all of vibrators. The composite signal forms a separable source vibrator record that allows for source separation through data inversion. Similarly, the same multiple vibrators may be activated independently and with or without encoding of the signal and then using the receivers and a recording instrument to capture a composite signal of the contributions of the multiple vibrators.
This composite signal can be separated into individual source records through data inversion or other more modern separation techniques like Adaptive-Subtraction. As an example, the ZenSeis® method uses the inversion technique and is covered under patents U.S. Pat. Nos. 7,295,490, 7,864,630, 8,004,931, 8,467,267 and 8,893,848, each of which is incorporated by reference herein in its entirety for all purposes.
Vibrators work on the principle of introducing a user-specified band of frequencies, known as the sweep, into the Earth and then cross-correlating that sweep function with the recorded data to define reflection events. This is normally called the “vibroseis” technique or method. The parameters of a vibrator sweep are:
i. Start frequency
ii. Stop frequency
iii. Sweep rate
iv. Sweep length
v. Gain or boost rate
A vibrator can do an upsweep that starts with a frequency as low as 1 to 2 Hz and stops at a high value of 80, 100, or 120 Hz. Alternatively, vibrators can do a downsweep that starts with a high frequency and finishes with a low frequency. Most Vibroseis data are generated with upsweeps to avoid ghosting problems in the subsequent correlation step.
The sweep rate can be linear or nonlinear. A linear rate causes the vibrator to dwell for the same length of time at each frequency component. Nonlinear sweeps are used to emphasize higher frequencies because the vibrator dwells longer at higher frequencies than it does at lower frequencies.
Sweep length defines the amount of time required for the vibrator to transverse the frequency range between the start and stop frequencies. As sweep length is increased, more energy is put into the Earth because the vibrator dwells longer at each frequency component. Sweep length is usually in the range of 2 to 40 seconds or longer.
If a vibrator sweep is 12 seconds long, then each reflection event also spans 12 seconds in the raw, uncorrelated data in the vibroseis method. It is not possible to interpret uncorrelated vibroseis data because all reflection events overlay each other and individual reflections cannot be recognized.
Gain or boost rate is the amount of extra time spent sweeping at different frequencies as a function of time. A linear sweep changes frequencies as a uniform function of time so each frequency band has the same amount of time spent on it. A positive boost or gain sweep spends more time in the later parts of the survey, which in an upsweep are the high frequencies. Alternatively, a negative boost or gain sweep spends less time in the later parts of the survey, which in an upsweep are the high frequencies. The opposite holds true in a downsweep so more time is spent in the low frequencies or less depending on the sign of the gain. Gain or boosts are normally expressed in terms of dB and usually range from 3 to 12 dB.
The data are reduced to an interpretable form by a cross-correlation of the presumed known input pilot sweep with the raw data recorded at the receiver stations. Each time the correlation process finds a replication of the input pilot sweep, it produces a compact symmetrical correlation wavelet centered on the long reflection event. In this correlated form, vibroseis data exhibit a high signal-to-noise ratio, and reflection events are robust wavelets spanning only a few tens of milliseconds.
As a general observation, if an area is plagued by random noise, vibrators are an excellent energy source because the correlation process used to reduce the vibrator sweep to an interpretable form discriminates against noise frequencies that are outside the source sweep range. Plus, if several sweeps are summed, any disorganized noise in the sweep range is attenuated by the power of summation or stacking of the data. However, if coherent noise with frequencies within the vibrator sweep frequency range is present, then the correlation process may accentuate these noise modes.
The duration of a vibroseis survey is largely determined by the long signal sweeps of the vibroseis source (typically 10-30 s). These long sweeps are required to obtain the necessary signal-to-noise ratio, but they also make vibroseis surveys time-consuming. To reduce survey time, methods have been developed to deploy various vibroseis groups simultaneously, based on transmitting specially encoded source sweeps. Codes have been designed such that the interfering source responses can be separated in a preprocessing step. Some of the more common simultaneous vibroseis recording methods are known as slip-sweep (Shell), ZenSeis® phase encoding (ConocoPhillips), Independent Simultaneous Sources or “ISS” (British Petroleum), flip-flop, orthogonal sweeps, cascading, upsweep-downsweep, etc., and combinations thereof. Additionally to these methods, new separation methods are being developed that depend on acquisition design parameters optimized for the separation of simultaneous or near simultaneous vibroseis sources.
However, all of these methods are still wedded to the idea of a sweep, which is really a function of the original equipment not allowing the generation of more complex signals. Vibroseis trucks use hydraulic motors to shake a baseplate, with the force opposed by a heavy weight mounted on the vehicle, and the ability to generate more complex signals with this hydraulic vibrator is very limited.
However, we have developed an electric seismic source with greatly increased capacities to vary the signal. U.S. Pat. No. 8,893,848 describes an electrically driven source wherein an acoustic energy delivery system comprises a frame carrying a number of linear motors. Each linear motor includes a tubular body and a rod or actuation bar positioned within the tubular body that extends telescopically from the lower end of the tubular body.
In operation, the frame is lowered into proximity to the ground G and the linear motors are operated to lower the replaceable feet into contact with the ground G. Once all of the replaceable feet are in contact with the ground G, the linear motors are activated to thrust the rods toward the ground G and deflect the ground G, thereby delivering an impulse into the earth. Since the linear motors are individually controllable, the ability is now available to develop seismic surveying methodologies that are no longer bound to the sweep principle.
Thus, there exists a need for developing methods for generating unique vibratory source acoustic signals that can be easily differentiated from one another, with high signal to noise ratios and without interference from harmonics, coherent noise, and the like. This application addresses one or more of these improvements.